How do you get a scraggy to learn knock off?

To get a Scraggy to learn Knock Off, you need to level it up to at least level 20. At that point, it can learn the move through leveling up, as Knock Off is part of its move set. If you want to teach it Knock Off earlier, you can use a TM if available in the game you are playing. Make sure to check the specific game mechanics as they may vary.